A. Field of the Invention
The invention relates to silicon carbide semiconductor devices, which use silicon carbide as a semiconductor material, and a method for manufacturing such silicon carbide semiconductor devices.
B. Description of the Related Art
Technical advantages for high frequency control and high power control have been offered conventionally by various techniques in a power semiconductor device (hereinafter referred to as a power device) using silicon (Si). On the other hand, there are situations in which the power device using Si cannot be used, such as in a high temperature environment or in an environment in which there are radioactive rays. Because of this, a power device having higher performance than the power device using Si has been developed.
As a material of such a high-performance power device, silicon carbide (hereinafter referred to as SiC) has been studied. A band gap of SiC is wider than that of Si, for example, 3.26 eV for SiC of 4H type (hereinafter referred to as 4H—SiC) and 3.02 eV for 6H—SiC. Because of this, SiC is superior in controllability of conductivity in the high temperature environment, and in resistance to radiation. In addition, SiC can be applied to a high breakdown voltage device because its dielectric breakdown voltage is higher than that of Si by about one order of magnitude. Further, SiC is suitable for high frequency control and high power control because an electronic saturation drift velocity of SiC is about 2 times of that of Si. There are various crystal polymorphs (polytypes) in SiC, and 4H—SiC in the polymorphs attracts attention as a material for the power device which has especially superior physical properties.
On the other hand, there are a lot of crystal defects and dislocations in single crystal SiC, and it is thought that these crystal defects and dislocations give characteristics of a SiC device a bad influence. As a representative large-sized defect of 4H—SiC, a micropipe defect is known. The micropipe defect has the Burgers vector of magnitude of more than the 3 c (3 times of interatomic distance of C-axis direction), and is a hollow defect that penetrates through in the C-axis direction. The breakdown voltage of the device is decreased remarkably due to the micropipe defect.
A technique to blockade the micropipe defect by epitaxial growth is reported to prevent the device from decreasing the breakdown voltage due to the micropipe defect. However, the technique resolves the micropipe defect which is a screw dislocation having the Burgers vector of Nc (N≧3) into a screw dislocation having the Burgers vector of less than or equal to 2 c, and it is not a technique in which the dislocation itself is removed.
In addition, as other large-sized defect of 4H—SiC, a carrot defect is known. The carrot defect occurs by a screw dislocation and a basal plane dislocation (hereinafter referred to as BPD). It is reported that defect density can be reduced by performing epitaxial growth in high temperature. In addition, it is known that the BPD changes direction at the boundary face between an epitaxial film and a substrate, and is converted into an edge dislocation.
In addition, in Japanese Patent No. 3462506 (line 41 of left column to line 1 of right column on page 9), of which the corresponding foreign Patent Application is International Publication No. WO 97/47045, aluminum (Al), boron (B) or gallium (Ga) is given as a dopant when a p-type region of a semiconductor device is formed. Nitrogen (N) or phosphorus (P) is the dopant when an n-type region is formed. In addition, Japanese Patent Laid-Open No. 61-291495 (claims and embodiment on page 3) discloses a technique in which two kinds of dopants (impurities) are used when a SiC film is grown on a semiconductor substrate.
Although according to the prior art, generation of the large-sized defects, such as, for example, micropipe and carrot defects, can be decreased, it is difficult to decrease the BPDs. The BPD causes a stacking fault, and it might cause fluctuation of a forward voltage and generation of a leakage current of the SiC device. Thus, it is important that the BPDs are decreased to improve characteristics of the SiC device.
In addition, although kinds of dopants to form the p-type region and the n-type region have been described in Japanese Patent No. 3462506, it has not been shown that defects can be reduced when these dopants are used, or in what kinds of conditions. In addition, the generation of defects is not considered in Japanese Patent Laid-Open No. 61-291495, and there is no disclosure whether the BPDs can be reduced by using the dopants in the description.